Sediment Transport And Dynamic Geomorphology Process In The Yellow River Estuary And Its Adjacent Sea

The Yellow River delta was developed by the sediment load delivered by Yellow River into Bohai Sea since1855. Due to the human activities or natural causes, the Yellow River course was shifted frequently in the delta over the past150years. In different distributary course, the Yellow River sediment into the sea built up the large sand spit in the estuary. Once abandoned, the sand spit was eroded by ocean dynamic. It can be inferred that the evolution of the Yellow River delta was the result of interaction between land and ocean. The Yellow River estuary and its adjacent sea are controlled by different water dynamic with development since1953. The present estuary was river-dominated, and the Diaokou and Shenxian promontories are wave-tide-dominated. Therefore, the process of sediment transport and sediment dynamic is different in different sea area, what's more, the sediment load delivered into the sea decrease dramatically since1980s due to the human activities and decrease in basin precipitation. These factors resulted in the spatial and temporal variability of dynamic morphology evolution.The hydrological and sediment observations were conducted in the Yellow River estuary and its adjacent sea, and the samples of suspended sediment, surface sediment and short length core were collected in the sea area form2004to2011. Base on these data, the dynamic sediment characteristic, sediment environment, transport process of suspended sediment and its dynamic mechanism were studied systematically in the Yellow River estuary and its adjacent sea. Additionally, the spatial and temporal characteristics of morphology evolution in the study area were analyzed based on the satellite remote data of modern Yellow River delta and the bathymetry data in the subaquatic delta. Furthermore, the controlling effects of sediment diffusion into the sea, sediment transport in coastal area and sediment dynamic process on the evolution of coast in the study area. These results can be concluded as follows: The surface sediment type is dominated by clayey silt in the study area. The content of sand is more in the area above～5m isobaths, and the contents of silt and clay are more in the area below～5m isobaths. Because of scouring and sorting by wave and tide, the grain size decreases from shore side to ocean side. As to the temporal change of surface sediment in the study area, the silt fraction increase but both clay and sand fraction decrease with time gonging on. In all, the grain size of surface sediment in the shore side increases, and the grain size in the deep water area decrease with time. The sediment environment was studied by method of End member model in the respective of dynamic component. The results indicate that three end members can reflect the sediment dynamic environment in study area. The distribution of end members in study area corresponds to the distribution of clay, silt and sand faction, and the end member represents different dynamic component in different sea area.From the respective of distribution of water dynamic in the sea area from Diaokouhe to Shenxiangou, the tide velocity is bigger in the sea area off Shibajing, and it is relative small in the sea area off Diaokouhe and Wuhaozhuang. The residual current of Euler and Lagrange is bigger in the sea area off Shibajing and Diaokouhe, and the residual current of Stokes is bigger in the sea area off Diaokouhe. As to the sea area from Gudong to present estuary, the tide velocity and the residual current of Stokes is big in the present estuary, and the tide velocity and residual current of Euler and Lagrange is small in the sea area off Gudong. The distribution of net water flux per width is consistent with the distribution of residual current of Lagrange in the study area.From the respective of the distribution of suspended sediment in the sea area from Diaokouhe to Shenxiangou, the suspended sediment concentration was high in the sea off Diaokouhe and Shibajing, and suspended sediment concentration was low in the sea off Wuhaozhuang. And the suspended sediment concentration was high in the shallow water and low in the depth water because the influence of wave dynamic on surface sediment is strong in the shallow water. Additionally, the surface sediment resuspends under the influence of tide current in this sea area, but the last time of resuspension is short. As to the sea area from Gudong to present estuary, suspended sediment concentration is high in the present estuary and low in the sea area off Gudong. And the sediment resuspension occurs in the present estuary because of big tide velocity, but the capability of sediment resuspension is weak under the influence of tide current. The hydrodynamics and transport process of suspended sediment is analyzed with comparing their characteristics in and after the water-sediment regulation period in July,2009. The results show that the process of water-sediment regulation intensifies the ebb dynamics but weakens the flood dynamics, and facilitates the formation of tidal shear front. And during the period of water-sediment regulation, the vertical salinity gradient increases and the water stratification is evident in the estuary; additionally, the suspended sediment concentration is high, and the sedimentation process is dominant in the transport process of the suspended sediment. After the water-sediment regulation, the suspended sediment concentration decreases greatly and is in direct proportion to current velocity; what's more, the sediment re-suspension is predominant in the process of suspended sediment transport. Furthermore, during the transport process of suspended sediment in the study area, the sediment transportation by advection current, tidal trapping and net vertical circumfluence is remarkable, and the advection current plays in key role in the sediment transportation, but the influence degree of these factors is different in different sea area.The change of coastline of Yellow River delta was influenced by estuary, water discharge and sediment load into the sea, ocean dynamic and human activities. The change of coastline of most area in the delta is result of interaction between sediment load into the sea and ocean dynamic. Otherwise, the coastline in some area change slowly because the construction of levee protecting the alongshore, and it extend towards sea due to the human activities including the enclosing tideland for cultivation and building shrimp and fish ponds and salt fields. The spatial and temporal changes of evolution of subaqueous delta are clear. The spatial change is characteristic of regional and switching between scouring and silitation, and the temporal change is characteristic of phase nature. In all, both the erosion rate in the north area of subaqueous delta and silitation rate in the south area decrease with time, and the evolution of subaqueous delta have been developing into dynamic equilibrium of scouring and silitation.The evolution of dynamic morphology of the Yellow River delta is influenced by the sediment into sea, sediment transport and sediment dynamic process. In the abandoned Diaokouhe and Shenxiangou area, the evolution of subaqueous delta is mainly shaped by wave and tide current. In the alongshore area, the movement of sediment is actively because of strong disturbance of wave dynamic, resulting in the strong erosion of slope below water; the resuspension sediment is transported by tide current towards depth water, causing the silitation in the depth water. The evolution of subaqueous delta in the present estuary is influenced by the sediment delivered by river into sea and sediment diffusion. Large amount of sediment delivered by Yellow River into the sea is deposited in the estuary, resulting in the strong silitation in the subaqueous delta especially in the front slope. Because the barrier effects of tidal shear front in the present estuary, the sediment delivered into the sea is mainly deposited in the sea area above～10m isobaths, and some suspended sediment diffuses northwestern under influence of residual current; in the abandoned old estuary, the subaqueous delta is eroded by wave dynamic.